1. Field of the Invention
This invention relates in general to devices for controlling a cursor on a display screen, and relates more particularly to such devices having a simplified construction and improved resolution.
2. Description of the Prior Art
A control ball device, commonly known as a "trackball", is often used with a computer to control the motion and position of a cursor on a video terminal screen. In typical applications, the cursor is moved to a desired location on the screen through movement of a roller ball by the user, and a specific function is then selected by pressing one or more keys on the control device.
A trackball device is described, for example, in U.S. Pat. No. 4,575,086, which shows a user-operated roller ball supported on three rollers, with two of the rollers mounted at right angles to each other and having encoding disks on their shafts. The encoding disks are provided with apertures which cooperate with a light emitting diode and a photodetector to provide a series of pulses as the rollers rotate. The control ball can be moved in both coordinate directions, with one of the encoding disks recording the motion of the ball in each of the coordinate directions. Hence, the cursor can be moved both up or down and to right or left on the display screen. The total device (control ball, rollers, encoding disks and electronics) is enclosed in a housing, the top side of which has a circular opening through which a part of the control ball protrudes. The operator can manipulate the control ball by placing his fingers on the protruding part of the ball.
U.S. Pat. No. 4,801,931 discloses a trackball device employing a pair of orthogonally displaced concave roller members which have the shape of truncated double cones and which carry encoding disks. These roller members engage the control ball and rotate in response to movement of the control ball to provide an indication of the control ball movement in the X and Y directions. Support for the roller ball in the housing is provided by one or two additional rotatable roller members which engage the roller ball at a point or points spaced from the roller members which provide the X and Y position information.